Ink mist is the term popularly applied to airborne droplets of ink ejected from press distribution systems and other rotating rollers. The mist is not only extremely annoying to personnel, but it also contaminates the pressroom and in some instances becomes a serious fire hazard as well as a major problem because of its health hazard potential in the working environment. Indeed, ink mist is one of the major factors limiting the speed of newspaper printing.
It is generally accepted that the misting phenomena is a direct consequence of the ink transfer process. When two rollers containing ink are rotated on contact, ink splitting takes place with the formation of filaments which elongate and then rupture. When the filaments break in more than one place, the free fragments coalesce into droplets and may be propelled into the atmosphere. Accordingly, the shorter the filaments produced on splitting, the smaller should be the number of mist particles generated. This assumption is the basis for mist reduction through alterations to the rheological properties of ink.
It has been established that, in the process of formation, ink mist acquires an electric charge, and both positively and negatively charged particles may exist in the mist. It has also been established that the magnitude of the charge of the particles of a given ink depends primarily on the rate of film separation and therefore directly on the press speed.
Press speed is the most important factor in ink misting. At lower press speeds, ink misting is usually negligible. At intermediate speeds, i.e. above 2,000 feet per minute, misting is a serious problem in newspaper printing. At speeds exceeding 2,500 feet per minute, misting becomes the major limiting factor. In this connection, it should be noted that changes in ink characteristics required for printing satisfactorily at high speeds render the ink more prone to misting.
Surveys of literature and prevailing practice regarding the misting of printing inks exist in the prior art, e.g. "Misting of Printing Inks", by Fetsko and Lavelle, American Ink Maker, March 1979, p. 47 et seq.; "Ink and Paper in the Printing Process", by Voet, Interscience Publishers, N. Y. (1952) pp. 79-86; "Ink Troubleshooting Tips", American Printer (1982) pp. 40-45; pp. 37 and 39; "The Problem Of Ink Fly" by Bryan, The Canmaker, (Oct. 1988); U.S. Pat. Nos. 3,963,501 and 4,321,094, etc.
Adjusting press operating variables, e.g. temperature, humidity, ink film thickness, roller settings, etc. have resulted in only limited success in achieving reduced misting. Furthermore, it is known that while additions to inks such as pigment loading, amine-treated montmorillonites, foam suppressors, water, electrolytes, amines and amides and asbestos have had some beneficial effect on reducing ink misting, these various methods do not permit high speed printing without concomitant misting.
It should be understood that any materials added to printing ink formulations to reduce misting must, at the same time, not adversely affect the rheological and lithographic properties of the ink since the quality of the final print depends greatly upon such rheological properties. Ink distribution and transfer, misting, print sharpness and clarity, print uniformity and density, penetration, rub resistance, piling and scumming are all related to the rheological characteristics of the ink used. Thus materials such as kaolin are not entirely satisfactory as mist-reducing additives because high shear viscosity buildup results in poor transferability and high loading of such types of clays interact with fountain solutions resulting in scumming. It has also been found that cationic surfactants which are frequently used for reducing misting of printing inks suffer from the disadvantage that they encourage improper emulsification and also result in deposits on the rollers when printing at high speeds is attempted, since most rollers are negatively charged and the cationic surfactant is attracted to them. Most anionic surfactants have been found to be inefficient for solving the misting problem.